CBD-681: Make things clearer

Adding a comment to make the implementation a bit clearer.

Change-Id: I0841c708c4808a4d4d3a235029570f50cccc6222
Reviewed-on: http://review.couchbase.org/22985
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>